SPX Index is between 1390 and 1365 at expiration
Buy 1 SPX 1390 Put at $25 

With the SPX exercise settlement value exactly at the strike price of $1390 at expiration, the 1390 put would be atthemoney and have no value.
With SPX at the breakeven point of 1365 at expiration, the 1390 put would be inthemoney and exercised.
The cash settlement amount received upon exercise would be:
$1390 (put strike price) – 1365 (settlement value) = $25 x $100 = $2,500
This amount of $2,500 is the total cost of the put.
If the SPX exercise settlement value is between 1390 and 1365 at expiration, the 1390 put will also be inthemoney and would be exercised. The cash settlement amount received, however, would be less than the total cost of the put, resulting in a partial loss for the position.
For example, say the exercise settlement value is 1375 at expiration. The cash settlement amount received upon exercise would be:
$1390 (put strike price) – 1375 (settlement value) = $15 x $100 = $1,500
SPX did decline in value, but not as much as anticipated. The put that originally cost a total of $2,500 is now worth $1,500, so the investor can recoup some of its initial purchase price and realize a partial loss.
$2,500 total premium paid for put
– $1,500 cash settlement amount received at put’s exercise (or sale at intrinsic value)
$1,000 partial loss